Nuclear magnetic resonance measurement techniques require knowledge and control of the spatial and temporal properties of magnetic fields in and proximate to the sensitive volume of the instrument. Most modern NMR instruments employ transient techniques. When the magnetic field itself exhibits transient characteristics there are accompanying parasitic consequences of eddy current excitation in conducting structures of the apparatus. The eddy currents contribute an increment to the field directed in opposition to the transient, with time constants which depend upon the structure supporting the eddy current.
In the prior art it is well known to provide a delay time after a transient or to so control the time dependence of the transient as to reduce or minimize the effect of the ensuing eddy current. This is of rather limited application given other requirements for time dependence and inter-pulse delays. It is also known in the prior art to provide for magnetic shielding to reduce the coupling of the magnetic field arising from the transient current to the surrounding structures wherein the eddy currents are induced. Shielding is is no more than partially effective, at best.
It is also known to compensate the effect of eddy currents by pre-compensation, that is, a deliberate distortion of the time dependence of the applied transient so as to yield the desired time dependence for the resulting field. The prior art is fairly reviewed and summarized by Jehenson, et al, J. Mag. Res. V.90, pp. 264-278 (1990); van Vaals, et al, J. Mag. Res., vol. 90, pp 52-70 (1990); Morich, et al, IEEE Trans. Mag. Imag., vol. 7, pp. 247-254 (1988).
The desired pulse shape for a field B in the direction p may be designated B.sub..rho. (t) and would ideally be supplied by a voltage waveform V(t) applied to coils of the appropriate orientation. We shall refer to V(t) as the "demand" pulse. In the real world, surrounding conducting structures such as nested cryostat shields, reservoir and containment vessel, probe shields and the like will have eddy currents induced thereon. These eddy currents generate magnetic field elements which are generally opposed to the field ideally associated with the demand pulse, as well as elements having diverse orientations. The time dependence of the realized pulse B.sub..rho. (t) is distorted relative to the time dependence of the demand pulse due to the superposition of the time constants for decay of the eddy currents on the various conducting structures. Moreover, as a consequence of eddy currents circulating on non-planar conductors, there will be eddy current induced transient field components directed along axes other than .rho..
More than one magnetic field component can be combined as a superposition to create spatial variations (gradients) in the resulting field. Excitation of a field element along .rho. may produce a desired gradient along p or a gradient along some other axis. There exists many combinations of desired gradient direction and field direction, and for each combination, there is potential for undesired eddy current distortion.